1. Field of the Invention
The present invention pertains to etching of carbon-containing silicon oxide films.
2. Brief Description of the Background Art
Low k dielectric materials are the trend for the next generation of dielectric materials used in semiconductor manufacture. The term xe2x80x9clow k dielectricxe2x80x9d typically refers to any material having a dielectric constant (k) value lower than that of silicon dioxide (k≈4.0). There are many different kinds of low k dielectric materials, including organic polymer-based materials, as well as silicon oxide-based materials which contain additives such as fluorine, carbon, and hydrogen.
Various methods for producing carbon-containing silicon oxide films are known in the art. U.S. application Ser. No. 09/021,788, of Yau et al., and German Patent No. DE 19654737, of Itoh et al., for example, disclose methods for producing such films. In particular, the ""788 application discloses a method for depositing a low dielectric constant film by reacting an organosilane compound (such as methyl silane, CH3SiH3) with an oxidizing gas (such as N2O or O2). During deposition of the film, methyl (CH3) groups bond to the silicon oxide structure. The presence of carbon in the film causes a reduction in the dielectric constant of the film.
Plasma etching of silicon oxide films is typically performed using a source gas containing a CxFy gas, such as CF4 or C2F6. The carbon and fluorine in the source gas are typically sufficient to provide an acceptable etch rate for the silicon oxide. However, for carbon-containing silicon oxide films, etching cannot proceed very far when CxFy is used as the sole etchant gas. The carbon in the film, together with the carbon and fluorine in the plasma source gas, combine to produce undesirable long-chain carbon-fluorine polymers which deposit over surfaces of the semiconductor structure being etched, hampering the etching process. The long-chain carbon-fluorine polymers also contaminate surfaces within the process chamber.
Prior art solutions to this problem have included adding a source of oxygen to the CxFy gas. The oxygen reacts with the carbon in the carbon-containing silicon oxide film, preventing undue polymer deposition and increasing the etch rate of the film. However, oxygen also attacks the overlying photoresist layer, which is typically used as a patterning mask for underlying feature (such as a contact via) etching. Therefore, although the presence of oxygen improves the etch rate of the carbon-containing silicon oxide film, the selectivity for etching the silicon oxide film in preference to an overlying photoresist layer reduced.
It would be desirable to provide an effective method for etching carbon-containing silicon oxide films that would provide both an acceptable etch rate and acceptable selectivity for etching the silicon oxide film relative to an overlying photoresist layer.
We have discovered a method for plasma etching a carbon-containing silicon oxide film which comprises exposing the film to a plasma generated from a source gas comprising NH3 and CxFy. The carbon-containing silicon oxide film generally comprises less than about 20 atomic percent carbon; typically the carbon content ranges from about 8 to about 20 atomic percent carbon; more typically, the carbon content ranges from about 8 to about 13 atomic percent carbon. The carbon-containing silicon oxide film frequently contains hydrogen. When hydrogen is present, the hydrogen concentration is generally less than about 45 atomic percent of the overall film composition. Typically the hydrogen concentration ranges from about 30 to about 45 atomic percent; more typically, the hydrogen content ranges from about 30 to about 40 atomic percent of the carbon-containing silicon oxide film.
Active fluorine species in the etchant plasma react with silicon in the carbon-containing silicon oxide film (substrate). Oxygen species generated from the carbon-containing silicon oxide film and hydrogen species from the plasma react with carbon. The method of the invention provides excellent etch rates, as well as superior etch selectivity for the carbon-containing silicon oxide layer in preference to an overlying photoresist masking material. It is our opinion, but not by way of limitation, that etch selectivity is improved because a layer of polymer (generated from the CxFy in the plasma source gas) is deposited upon the upper surface of the photoresist masking layer during the etching process. This layer of deposited polymer protects the photoresist from being consumed during the silicon oxide etching process. Simultaneously, the ammonia (NH3) gas portion of the plasma source gas functions to xe2x80x9cclean upxe2x80x9d deposited polymer on the photoresist surface, on the etched surface, and on the process chamber surfaces. It is necessary to achieve the proper balance between the CxFy and NH3 in the plasma source gas in order provide a balance between by-product polymer deposition and removal on various surfaces of the substrate being etched. Oxygen may be added to the plasma source gas to increase the etch rate of the carbon-containing silicon oxide, but this reduces the selectivity in favor of etching of the carbon-containing silicon oxide relative to the photoresist.
Carbon and nitrogen are typically present in the source gas in an atomic ratio within a range of about 1:0.3 to about 1:3 of carbon:nitrogen; preferably, within a range of about 1:0.7 to about 1:2.2 of carbon:nitrogen; and more preferably, within a range of about 1:1 to about 1:1.8.
The method of the invention comprises exposing the carbon-containing silicon oxide film to a source gas comprising NH3 and CxFy, where x ranges from about 1 to about 6 and y ranges from about 4 to about 8. Typically, x=2 to 4 and y=4 to 8. In particular, we have found that C2F6, C4F6, C4F8, and C5F8 provide excellent etch rates and etch select
Although less preferred, the plasma source gas may further include a non-reactive, diluent gas selected from the group consisting of argon, helium, xenon, krypton, and combinations thereof.
The method of the invention for etching carbon-containing silicon oxide films has provided etch rates of at least 2.2 xcexcm per minute and etch selectivity relative to an overlying photoresist layer of up to about 25:1. This combination of rapid etch rate of the carbon-containing silicon oxide with high etch selectivity relative to the photoresist masking layer was unexpected. Also important is the clean process chamber surface after completion of etch.